1. Field of the Invention
The present invention relates generally to an improved method for optimizing a hydraulic fracture treatment of a subsurface formation, and more specifically relates to an improved method for determining the characteristics of the fracture in real time from fracturing fluid pressure and volume data, and optimizing the fracture treatment based on those determined characteristics.
2. Description of the Related Art
It is common in the industry to stimulate hydrocarbon bearing subsurface formations through hydraulic fracture operations. Typically, a fracture treatment consists of blending special chemicals to create an appropriate fracturing fluid and then pumping the fracturing fluid into the hydrocarbon bearing formation at a high enough rate and volume to cause the hydrocarbon bearing formation to fracture. Often times the fracture treatment consists of two different fluids used one after the other. The second fluid typically contains a propping agent, or proppant, which functions to prop open the fracture.
Hydraulic fracturing has evolved from the simple, low volume, low-rate treatments of the early 1950's into the complex procedures currently used. Today, hydraulic fracturing is the most widely used process for stimulating production from oil and gas wells.
The goal of a fracture treatment is to produce a subsurface fracture in a hydrocarbon bearing formation that is propped open with the right amount of proppant in the right locations. Fracture shape affects the production rate of the well and the production life of the well. The importance of early detection of deviations from the ideal fracture shape, or other deleterious occurrences such as, for example, screen out, vertical extension, or out of zone fracture, is well known in the art in order to improve or optimize the fracture treatment.
Recent advances in early detection of these conditions during fracturing treatments can be traced to advances in the interpretation of downhole fracturing pressures during fluid injection. These advances in interpretation have provided methods for controlling undesirable vertical fracture growth, improving fracture conductivity, and reducing formation damage.
An advance in the interpretation of downhole fracturing pressures was made by Nolte and Smith who found that fracture extension rates, critical net fracturing pressures, and vertical growth behavior can be inferred from downhole fracturing pressures. Nolte and Smith, Interpretation of Fracturing Pressures, JPT 1767-75 (Sep. 1981). Others in the hydraulic fracturing art have suggested that various types of fracture behavior can be identified from downhole fracture pressure information. Basically, these prior art techniques have compared the downhole fracturing pressure against fluid injection time on a logarithmic basis.
Various guidelines have been presented by those skilled in the art for interpreting the logarithmic behavior of fracturing pressure and injection time. One assumption implicit in most, if not all, of the interpretive guidelines is that the fluid injection rate is constant. However, injection rates are never really constant. For example, fluid injection rate does not instantaneously reach the desired value when pumping starts, but increases continuously from zero to the desired rate.
Rate variations may also occur as equipment is brought off and on line during the course of a treatment. And even when the rate is held arguably constant, mechanically induced variations in the injection rate still exist. The overall effect of this non-constant rate of injection is to lessen the accuracy and reliability of these prior art techniques.
Thus, it has long been desired to develop a method and/or apparatus for optimizing fracture treatment programs that accurately and reliably characterize the actual fracture regardless of variations in fluid injection rate.
The present invention answers this need by providing an improved method for characterizing the actual parameters of the hydraulically induced subsurface fracture from fracturing fluid pressure and volume data, and optimizing the remainder of the fracture treatment, or a subsequent fracture treatment, based on the fracture parameters.